Oxygenation plays a critical role in health and medicine. Deficits or excesses in cells, tissues and organisms are associated with disease, damage, poor healing, and even death. For example, hypoxia is implicated in cancer progression, metastasis, and resistance to therapies. Hypoxia inducible factor 1 (HIF-1) expression in low oxygen environments accelerates angiogenesis and promotes tumor survival, and radiation and drug treatments can be less effective when reactive oxygen is not present to enhance the tumor damage or when hypoxia-induced changes in the tumor microenvironment otherwise affect drug delivery and action. Despite the importance of oxygen partial pressure (pO2) to biomedicine, its assessment is often limited to oxygen consumption rate (OCR) measurements, invasive probes, or expensive imaging modalities. Often methods provide only average values for whole populations of cells, or are restricted to single point measurements in space or time. The long-term goal of this project is to develop a versatile material set and platform of pO2 imaging technologies that can enhance preclinical studies and aid in diagnosis, treatment and surgery in many medical contexts. Building upon success in proof of concept studies and the strengths of our team in materials synthesis, imaging, and cancer biology, the objective of this application is to develop dualemissive difluoroboron ?-diketonate-poly(lactic acid) (BF2bdkPLA) dye-polymer nanoparticles (BNPs) in conjunction with imaging methodologies for luminescence detection and ratiometric O2 sensing in breast cancer in vitro and in vivo mouse models. This objective will be accomplished by pursuing the following specific aims: 1) BNP dyes will be chemically modified for broad range emission color tuning and oxygen sensitivity modulation; imaging methods will be developed for their use in in vitro ratiometric O2 sensing (i.e. both fluorescence (F) and phosphorescence (P) detection). 2) BNP polymers will be adapted for passive and active targeting for pO2 monitoring with improved spatial specificity; targeted BNPs will be investigated in vitro and in vivo. 3) BNP hypoxia imaging agents will be tested for their ability to detect tumors, and to monitor tumor progression and radiation and chemotherapy response over time in a mouse mammary window model. The proposed cost- effective technology is innovative because it enables dynamic hypoxia imaging with improved combined spatial and temporal resolution compared to existing approaches with a modular, tunable, synthetically accessible materials platform. The expected outcome is a versatile oxygen nanosensor technology in conjunction with common optical imaging modalities to quantify pO2 in cells, tissues and in vivo. BNPs with greater specificity, tissue penetration of light, and multiplexing capability will result. This work will have a positive impact on cancer car because it will better illuminate the relationships between hypoxia, cancer progression, and treatment protocols. Ultimately, BNPs will shed light on many medical challenges by helping to map relationships between oxygenation and biological function. This is already being realized.